The devices used in operations for the maintenance or repair of nuclear reactors or of their fuel assemblies and, more generally, in work carried out on sensitive installations conventionally comprise so-called positively acting mechanisms which ensure a high degree of operating safety of these devices.
In particular, devices for the maintenance or repair of nuclear reactors or of their fuel assemblies can comprise grasping means, such as tongs, the actuation of which is ensured by means of a positively acting mechanism.
Such a mechanism comprises a jack which can be fed with hydraulic fluid or with demineralized water and the rod of which is connected, at one of this ends, to the member being actuated or retained, and this member can, for example, consist of a piece or a clamping jaw of a pair of grasping tongs.
An elastic actuating means, such as a helical spring, comes to bear on a thrust piece fixed to that end of the rod of the jack with is opposite the end connected to the actuated member. The spring is arranged in a housing fixed relative to the body of the jack and maintained in a state of tension or compression, ensuring the actuation or retention of the member connected to the rod of the jack, when the jack is not fed with hydraulic fluid.
For example, if grasping tongs are involved, a movable jaw of these tongs can be retained in the clamping position by means of the spring, as long as the jack is not being fed.
In the event of failure of the jack, for example a breakdown in its feed, the positive action of the mechanism is not released.
The feed of the jack with hydraulic fluid makes it possible to displace its rod in the direction opposite to the direction of actuation of the elastic means, and thus to release the positive action of the mechanism. For example, where grasping tongs are involved, the activation of the jack makes it possible to unclamp the tongs by releasing at least one of its clamping jaws.
In the event of a breakdown making it impossible to actuate the jack, for example in the event of a breakdown in the feed of the jack with hydraulic fluid or with demineralized water, even though the operating safety of the device having the positively acting mechanism is not jeopardized, the fact that the mechanism cannot be released may cause problems, inasmuch as it is usually not possible to have access to the mechanism which is used for working on irradiated components under a particular head of water.